Anvari.org

Propeller Head: N. Used By Hackers, This Is Syn. With {computer Geek}.

:propeller head: n. Used by hackers, this is syn. with {computer
geek}. Non-hackers sometimes use it to describe all techies.
Prob. derives from SF fandom's tradition (originally invented by
old-time fan Ray Faraday Nelson) of propeller beanies as fannish
insignia (though nobody actually wears them except as a joke).
-- The AI Hackers Dictionary

Related:

Retrieved from http://www.anvari.org/fortune/Miscellaneous_Collections/211162_propeller-head-n-used-by-hackers-this-is-syn-with-computer-geek.html